1. Field of the Invention
The present invention relates generally to optical sensors used in monitoring respiration, and, in particular, to optical sensors that include an optical element formed from a material, or combination of materials, that are substantially transparent to wavelengths of electromagnetic radiation that would otherwise cause warming of the optical element.
2. Background of Related Art
Optical sensors are useful for identifying and quantifying substances, including contaminants, that are present in a gas sample. Typically, an optical sensor includes a housing that contains a source of radiation, commonly referred to as an emitter, and a detector that receives radiation. The emitter provides one or more wavelengths of electromagnetic radiation that are passed through the gas sample, either directly or indirectly. The electromagnetic radiation is received by the detector and the signal from the detector facilitates evaluation of the gas sample, for example, for identifying and quantifying at least one constituent of the gas sample. In addition, the housing includes one or more optical elements, such as windows or lenses, through which the monitoring radiation is emitted into a sample and through which the monitoring radiation exits the sample.
Many optical gas sensors are configured to evaluate the direct affects of the sample on monitoring radiation or of the monitoring radiation on the sample. The emitter of such a sensor is typically configured to direct monitoring radiation into the sample. The detector of such a sensor senses a change in intensity of the monitoring radiation resulting from absorption of the monitoring radiation by one or more constituents of the sample, or senses temperature changes that occur as one or more constituents of the sample absorb the monitoring radiation. When correlated with a certain wavelength of monitoring radiation, the change in intensity or temperature indicates that a specific substance is present in the sample. The amount of the change in intensity or temperature corresponds to the amount of that substance in the sample.
Another type of optical gas sensor employs a technique known as “luminescence quenching.” A luminescence quenching type sensor includes a luminescent material, e.g., a fluorescent or phosphorescent material, which is excited when exposed to monitoring radiation. When exposed to a certain substance, the intensity of luminescence of the luminescent material decreases, or is quenched. The degree to which the luminescence is quenched corresponds to the amount of the substance in the sample that causes the quenching.
The lenses and windows of optical gas sensors are typically fabricated from durable, scratch-resistant materials, such as sapphire. This is done to enable the optical elements to withstand the incidental contact to which the lenses or windows will inevitably be subjected during repeated use, cleaning, and storage.
FIG. 1 is a schematic representation of a portion of a conventional gas sensor 30 illustrating the opacity of a sapphire optical element 10, such as a window or lens, to wavelengths of warming radiation 20. While sapphire optical elements 10 have good transparency for visible light and near infrared wavelengths of electromagnetic radiation 22, which is typically referred to as “monitoring radiation”, they absorb longer, warming wavelengths of infrared radiation 20, which is referred to herein as “warming radiation.” The transparency of optical element 10 to monitoring radiation 22 is illustrated in FIG. 1 by showing the monitoring radiation passing through the window. The opacity of the optical element to warming radiation 20 is illustrated by showing the warming radiation as not passing through the optical element. For purposes of the present invention, “warming radiation” is radiation having a wavelength of at least about 6 μm.
Because conventional sapphire optical elements absorb warming radiation, a substantial portion of warming radiation 20 generated by an emitter of a sensor including such an optical element 10 is trapped by the optical element, thereby raising the internal temperature of the sensor to undesirably high levels. Optical gas sensors typically employ independent temperature control means to stabilize the temperature of their internal components. These undesirably high temperatures inside sensor 30 may interfere with such temperature control means, and affect the performance of sensor 30, degrading accuracy and long term durability.
While heating of an optical element can have undesirable consequences for an optical sensor, there are situations where absorption of warming radiation 20 has benefits. In respiratory monitoring, as well as when other gases and fluids are monitored, condensation, or “fogging,” occurs when a relatively warm sample, such as an exhaled breath, contacts a colder object, such as the window or lens of a sampling component (typically referred to as a “cell” or “cuvette”) of an optical monitoring system. Basically, as the portions of the sample that contact the window of the sampling component are cooled, water molecules in the form of vapor condense, fogging the window. Unfortunately, condensation, or fog, on components of the optical monitoring system can interfere with the monitoring process and adversely affect on the accuracy of the data that may be obtained with such systems. Absorption of warming radiation results in heating of the window, thereby reducing fogging, much like the defrost feature in an automobile heater.
The problem of condensation on the windows of optical monitoring components has also been addressed by various other approaches. One approach to reducing or eliminate fogging on the windows of sampling components involves de-humidifying the sample with a desiccating material, such as NAFION®. However, the inclusion of a de-humidifier in an optical gas sensor increases the complexity and cost of the sensor.
Other approaches have been used to heat the windows of the gas sensor directly. An example of such a conventional window-heating technique includes the use of an electrical heater to warm each window of a sampling component. Of course, power must be supplied to an electrical heater for it to work. Thus, additional circuitry must be added to the system, increasing the overall complexity and cost of a system that includes an electrical heater.